In conventional large bulldozers mainly used for digging, a blade device is mounted on a front portion thereof while a ripper device is mounted on a rear portion for digging or pushing soil. The ripper device used at a working site where the soil is mixed with hard rocks is mounted on a bracket fixed to the rear surface of a vehicle body through lifting and tilting hydraulic cylinders. The ripper device performs digging by inserting a blade thereof into the ground or by crushing the hard rock while changing the insertion angle. At a relatively soft ground, digging is performed by inserting the blade upright to the ground. At a hard rock ground, the operation of the ripper device is temporarily stopped and the rear portion of the body is floated. In that state, the hard rock is crushed utilizing the weight of the bulldozer and the pushing down force generated by a lifting cylinder of the ripper device. However, there is a limitation in the crushing of a hard rock, and crushing of the hard rock deteriorates the working efficiency. Hence, a ripper provided with a hydraulic breaker to crush hard rocks by giving a blow to it has been employed in recent years. Such a ripper is capable of crushing hard rocks which would be conventionally difficult to crush and can thus increase the digging efficiency.
However, at a working site where the aforementioned type of construction machine is used, the ground is generally composed of soft soils and hard rocks which are present in a mingled state, and the operator determines by intuition the digging condition and operates the bulldozer accordingly. That is, when the machine comes to a hard rock and the digging resistance is thereby increased, the operator crushes it by manually operating the vibrating mechanism of the impact ripper. When the digging resistance reduces, the operator stops the operation of the breaker. Such an operation dependent on the intuition of the operator requires the operator's skill and may generate wasteful striking of the ripper which may lead to breakage of the impact ripper. Thus, the working efficiency is deteriorated, and the operator often has fatigue.
In view of the aforementioned drawbacks of the conventional technique, an object of the present invention is to provide a method of automatically controlling an impact ripper which does not require the operator's skill, which is capable of reducing fatigue and of preventing breakage of the impact ripper caused by wasteful striking, which is capable of increasing the operability of the ripper during the operation of a breaker and which is capable of preventing an increase in the vibrations of a body during crushing of a rock bed.